The present invention relates to a method of manufacturing a printed circuit board for improving signal delay and impedance mismatching, and a printed circuit board suitable for the method of manufacturing the same. More particularly, the present invention relates to a method of shortening a stub of the printed circuit board.
In order to mount an electronic component, such as a LSI (large-scale integrated circuit), to a multilayer printed circuit board, a through-hole is formed and conductive plating is applied to the through-hole, to provide a terminal for connecting with a predetermined inner conductor-wiring layer. However, since a plated portion of the through-hole is longer than the distance to the target conductor-wiring layer, there arise problems in impedance mismatching, signal delay, and waveform distortion unless the overlong portion (hereinafter referred to as stub) is shortened.
Therefore, it is necessary to perform drilling (hereinafter referred to as back-drilling) from the back side up to just before the conductor-wiring layer to remove the stub of plating by use of a drill having a diameter slightly larger than that of the through-hole. There is an increasing need for this process with an increase of high-frequency printed circuit boards in recent years.
In the processing, controlling the depth of back-drilling has a problem. A printed circuit board is generally formed by compressing with heat resin layers and conductive wiring layers stacked alternately, which may cause a variation in the thickness of each layer and board, which results in a depth variation ranging from 60 to 100 μm of the position of the conductive wiring layer.
To cope with the above problem, the following method is devised. A detecting portion of a detection pattern is formed just before the target conductor-wiring layer and a voltage is applied between the detection pattern and a drill for back-drilling. Thereby a current flows when the drill comes in contact with the detecting portion, then the back-drilling is terminated. (for example, refer to JP-A-2006-173146 (FIG. 1, Detecting portion 35) or JP-A-2005-116945 (FIG. 7, Current detection layer 16-1))
However, with the above-mentioned conventional method of using detecting portions, a detection pattern becomes complicated and, accordingly, the number of layers increases, which may result in an increase in the material cost in some cases. Further, there may be a hole around which a detecting portion cannot be formed at all.